1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor useful for electrophotographic image forming apparatus such as copiers, facsimiles, printers and direct digital printing plate forming machines, and to a process cartridge and image forming apparatus using the photoreceptor. More particularly, the present invention relates to an electrophotographic photoreceptor including a charge transport polymer material and a filler, and to a process cartridge and image forming apparatus using the photoreceptor.
2. Discussion of the Background
As electrophotographic photoreceptors, the following photoreceptors are well known:
(1) photoreceptors in which a photosensitive layer including selenium or a selenium alloy as a main component is formed on an electroconductive substrate; PA1 (2) photoreceptors in which a photosensitive layer including an inorganic photosensitive material such as zinc oxide or cadmium sulfide and a binder resin is formed on an electroconductive substrate; PA1 (3) photoreceptors in which a photosensitive layer including an amorphous silicon compound is formed on an electroconductive substrate; and PA1 (4) photoreceptors in which a photosensitive layer including an organic photosensitive material is formed on an electroconductive substrate (hereinafter referred to as an organic photoreceptor or an OPC). PA1 (1) the maximum charge quantity decreases; PA1 (2) the dark decay increases; and PA1 (3) the photosensitivity deteriorates. PA1 (1) good abrasion resistance; PA1 (2) maintaining good charge properties even when repeatedly used for a long time, resulting in production of good images without undesired images such as black spots or white spots; PA1 (3) producing good images without image defects such as black streaks or white streaks even after the photoreceptor is preserved under high temperature/humidity conditions; and PA1 (4) high sensitivity and long life.
Among these photoreceptors, organic photoreceptors are widely used because of having advantages against other photoreceptors such that they have relatively low manufacturing costs and they are friendly to environment, and in addition there is a wide choice when a desired OPC is designed.
On the other hand, recently the development of information processing technology is remarkable. Electrophotographic image forming apparatus such as electrophotographic copiers and printers become more important as an information processing apparatus. In addition, needs for high speed image forming apparatus, miniaturized image forming apparatus, and high quality image forming apparatus increase more and more. Therefore, a need exists for an electrophotographic photoreceptor (hereinafter referred to as a photoreceptor) having good durability.
In electrophotographic image forming apparatus, processes such as charging, exposure, development, image transfer and cleaning are repeatedly performed. A photoreceptor used in such an electrophotographic image forming apparatus is subjected to various mechanical and chemical actions, and thereby the mechanical and electrostatic characteristics of the photoreceptor deteriorate. For example, abrasion and scratch are exemplified as the mechanical damages. In addition, as the electrostatic damages, the following damages are exemplified:
Such electrostatic damages are caused by oxidization of the photoreceptor constituents such as a binder resin, a charge transport material etc. due to ozone and NOx which are generated in the charging process.
In addition, when products, which are generated due to corona discharging performed in the charging process, adhere to a photoreceptor, the image qualities of images produced by the photoreceptor deteriorate.
As the size of image forming apparatus becomes small, the diameter of the photoreceptor drum used for the image forming apparatus also becomes small. When a small-sized photoreceptor is used, there are great risks such that the photoreceptor is easily abraded. This is because it is needed to use a relatively hard rubber blade for cleaning the surface of such a small-sized photoreceptor while applying relatively high pressure to the rubber blade. Therefore, the abrasion of the photoreceptor is accelerated, and thereby charge properties of the photoreceptor such as maximum charge quantity and photosensitivity change, resulting in formation of undesired images and deterioration of color reproducibility of the resultant color images.
In attempting to solve these problems, Japanese Laid-Open Patent Publications Nos. 1-205171, 7-333881, 8-15887, 8-123053, 8-146641 etc. have disclosed techniques in which a filler is included in a photoreceptor to reduce abrasion of the photoreceptor. However, when these photoreceptors are repeatedly used for a long time, the potential of an area of the photoreceptor, which is exposed to imagewise light, increases, resulting in deterioration of image qualities, such as decrease of image density. In addition, Japanese Laid-Open Patent Publication No. 8-179542 discloses a technique in which a protective layer is formed on the photoreceptor. However, when a protective layer is formed, resolution of the resultant images deteriorates, and therefore this technique is not satisfactory.
Japanese Laid-Open Patent Publication No. 59-223442 discloses a photoreceptor including a protective layer having two layers in which the concentration of a filler in the upper protective layer is relatively high compared to that in the lower protective layer, wherein the filler is electroconductive However, when an electroconductive filler is used in a protective layer of a photoreceptor, the charge formed on the photoreceptor diffuses if the photoreceptor is exposed to dot-shaped imagewise light, resulting in deterioration of image reproducibility. In particular, this phenomenon prominently occurs when such a filler is used for organic photoreceptors.
In addition, Japanese Laid-Open Patent Publication No. 63-305364 discloses a photoreceptor including a protective layer having two layers in which the concentration of a filler in the upper protective layer is relatively high compared to that in the lower protective layer, wherein the protective layers include a combination of an insulating silicon dioxide and a nylon/urethane resin. This protective layer has significant dependence on environmental conditions. Therefore, the photoreceptor has relatively low reliability.
Further, Japanese Laid-Open Patent Publication No. 8-101524 and U.S. Pat. No. 5,677,094 have disclosed a photoreceptor in which a charge transport polymer material including a structure in its polymer chain, which has a charge transport function, is used in a protective layer. In addition, a photoreceptor having a protective layer including a charge transport polymer material and a filler having high hardness is also disclosed therein. When the photoreceptor having a protective layer including a charge transport polymer material and a filler having high hardness is used, it is possible to control increase of residual potential and abrasion of the resultant photoreceptor so as to be relatively low compared to the former photoreceptors.
However, the photoreceptor has a drawback in that undesired images due to local charge leak of the photoreceptor when the photoreceptor is repeatedly used. For example, when a charged photoreceptor is developed with a toner having a charge whose polarity is the same as the charge formed on the photoreceptor (i.e., when a reverse development is performed), undesired black spots are produced in a background area of images. When a charged photoreceptor is developed with a toner having a charge whose polarity is opposite to the charge formed on the photoreceptor (i.e., when a normal development is performed), undesired white spots are produced in images.
In addition, such a photoreceptor having a protective layer including a conventional charge transport polymer material and a filler has a drawback in that when the photoreceptor is preserved in high temperature/humidity conditions or when the surface of the photoreceptor contacts paper, rubber, hands, fingers etc., cracks are generated in the surface portion of or inside of the photosensitive layer, resulting in production of undesired images (black streak or white streak images) corresponding to the cracks, or peeling of the photosensitive layer. In particular, when a belt-shaped photoreceptor having such a photosensitive layer is fed by a plurality of drive/support roller each having a small diameter, cracks are easily formed therein because the belt-shaped photoreceptor is bent in a small radius of curvature at the drive/support rollers. Therefore, the photoreceptor has a short life.
Because of these reasons, a need exists for an electrophotographic photoreceptor which has high sensitivity and long life, and can produce good images without image defects such as black or white spots and streaks even when repeatedly used under various environmental conditions.